The present invention relates to spline connections, and more particularly, to spline connections in which one of the sets of splines comprises crowned, external involute splines.
The present invention is applicable wherever a shaft is provided with a set of crowned, external involute splines which transmit torque to or from a set of straight, internal involute splines. The internal and external splines cooperate to form a universal connection, wherein the axis of the shaft having the external splines is not parallel to the axis of the element associated with the internal splines. Therefore, it should be understood that the present invention may be applied to a shaft having a single set of crowned, external splines on one end, and having a different form of universal connection means at its other end.
The invention is especially advantageous when used in a drive connection for transmitting torque between a member having pure rotational motion, and a member having both orbital and rotational motion. Such a connection is commonly found in rotary fluid pressure devices (either a pump or motor) which include a gerotor gear set, and the present invention will be described in connection therewith.
Fluid motors of the type utilizing a gerotor gear set (displacement mechanism) to convert fluid pressure into a rotary output have become popular and are especially suited for low speed, high torque (LSHT) applications. In most of the commercially available fluid motors of this type, one of the primary factors limiting the torque output capability of the motor is the strength of the drive connection which transmits torque from the orbiting and rotating member (rotor) of the gerotor gear set to the output shaft of the motor. Typically, this drive connection comprises one set of internal splines defined by the rotor, and another set of internal splines defined by the motor output shaft, and a main drive shaft (dogbone) having a set of crowned, external splines at each end thereof, in engagement with the sets of internal splines. The present invention will be described in connection with a main drive shaft of this type, having crowned, external splines at both ends, even though the invention is not so limited. In the drive connection described above, the internal splines are straight, whereas the external splines are crowned to take into account the "misalignment", or the eccentricity of the gerotor rotor relative to the axis of the output shaft.
Crowned splines are usually manufactured by a process referred to as "rise-and-fall" hobbing, especially when a large volume of production is required. When crowned, external splines are produced by this method, the set of splines normally defines a central plane, oriented perpendicular to the axis of the drive shaft, with the central plane intersecting each of the splines at the theoretical "peak" of the crown. As is well known to those skilled in the art, the crown angle, determined by the hobbing angle, of the crowned splines should be sufficient that, at any axial distance from the central plane along the external spline, there will be a clearance between the external spline and the adjacent internal spline to prevent interference between adjacent splines during operation of the device. Spline clearance is normally measured perpendicular to the flank of the external spline while the axis of the main drive shaft is oriented at its operating angle (running angle).
The present invention relates, more specifically, to the root radius of the external, crowned splines. The root radius is the radius of curvature of the root surface in the region of the central plane. In prior art crowned splines, little attention has been given to the selection of a root radius which is appropriate in view of the other design parameters of the spline connection. In most of the prior art fluid motors of the type described above, the root radius has been determined by one of two different approaches. In one case, the root radius actually approaches zero, i.e., the opposite ends of each crowned spline meet in a peak at the central plane. In the other design approach, which has been utilized commercially by the assignee of the present invention, the root radius is selected to be one-half of the minor diameter of the drive shaft, the minor diameter being the diameter of the root surface, measured at the central plane of the crown.
One aspect of the present invention is the recognition that the clearance between the internal and external splines is related to the proper selection of the root radius.